The story centers on a man named Train Heartnet who withdrew from an elite group of assassins called the Toki no Bannin (時の番人; the Chronos Numbers) to become a Sweeper (a bounty hunter).In Train's world, there is a secret organization, Chronos, that rules one third of the world through underground control of almost every kind of business. To maintain the balance of the world, Chronos is essential, and therefore, there is the need to have people to protect Chronos. Besides all normal erasers, there are special elite assassins with Orichalcum-made weapons called the Chrono Numbers. Chrono Numbers have their numbers engraved on their body and their weapon. There were twelve total Chrono Numbers, but when Train Heartnet, an unsurpassable shooter, decided to join Chronos in their cause, the number XIII was founded. No one that was targeted by Train Heartnet, also known as Black Cat, for bringing "bad luck", and his extremely quick movements and attacks, ever survived. The name Black Cat was the most feared among assassins.His only objective was "to be the best". In one of his missions, he had to kill a target, but the target's child was at the scene. Train, recalling his childhood trauma, could not kill the target in front of his child.Eventually, he met Saya Minatsuki, who was a sweeper. Through his contact with Saya, Train changed drastically, a change that culminated with leaving Chronos. However, a man who admired Train in an obsessive way, Creed Diskenth, did not accept that an elite assassin was being changed that way. Creed then set out to kill Saya, while Train was not there. Train reached the scene too late and tried to fight Creed, but he escaped.Two years later, Train is a Sweeper, traveling with his partner Sven Vollfied. Their life as sweepers is about to be changed when the present is invaded by the past: Creed Diskenth, after two years of being away, returns. And he wishes to meet Train, with a proposal to become his partner.